1. Field of the Invention
The present invention relates to methods and apparatus for efficiently transmitting downlink acknowledgement channel messages.
2. Description of the Related Art
A typical cellular radio system includes a number of fixed base stations and a number of mobile stations (user equipment). Each base station covers a geographical area, which is defined as a cell.
During an uplink transmission in the Third (3rd) Generation Partnership Project Long Term Evolution (3GPP LTE) systems, a unit of user equipment (UE) transmits a data packet to a base station (BS) after receiving an uplink scheduling grant (i.e., uplink grant) from the BS. In response to the received data packet from the UE, the BS transmits a downlink acknowledgement message (i.e., downlink ACK) to the UE. During a downlink transmission, a BS transmits a data packet to a UE after transmitting a downlink scheduling grant (i.e., downlink grant) to the UE. In response to the received data packet from the BS, the UE transmits an uplink acknowledgement message (i.e., uplink ACK) to the UE.
In the case of four transmission antennas (4Tx), the Space-Frequency Block Code combined with Frequency Switched Transmit Diversity (SFBC+FSTD) transmission matrix is adopted in LTE standard according to “Chairman's notes”, of Orlando 3GPP LTE RAN1 49bis meeting held in June 2007.
In addition, Code division multiplex (CDM) based Acknowledgement and Negative Acknowledgement (ACK/NACK) multiplexing and its transmit diversity scheme for two transmission antennas (2Tx) case has been proposed in “Performance of DL ACK/NACK transmission”, 3GPP R1-071805, published by Samsung in March 2007, and “Downlink ACK/NACK Transmit diversity”, 3GPP R1-073099, published by Samsung in June 2007, to multiplex the information bits of acknowledgement channel signals.
CDM ACK/NACK approach, however, does not immediately work with the 4Tx SFBC+FSTD transmit diversity scheme, because the CDM orthogonality is lost after the 4×SFBC+FSTD operation.